Lancet study casts doubt on COVID-19 herd immunity feasibility

PTI, Jul 7, 2020, 4:47 PM IST

New Delhi: A Spanish study published in The Lancet journal has cast doubt over the feasibility of herd immunity when enough people become infected with a virus to stop its spread as a way of tackling the COVID-19 pandemic.

For herd immunity to work, at least 70 percent of a population needs to be immune to protect the uninfected people, experts say.

The study of over 60,000 people estimates that around just 5 percent of the Spanish population has developed antibodies, according to the researchers, including those from the National Centre for Epidemiology, Spain.

The aim of the population-based study was to estimate the seroprevalence of the level of a pathogen in a population, as measured in blood serum of SARS-CoV-2 infection in Spain at the national and regional levels.

“About 35,883 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size,” the researchers said.

From April 27 to May 11, 61,075 participants answered a questionnaire on a history of symptoms compatible with COVID-19 and risk factors, among others.

“Prevalences of IgG antibodies were adjusted using sampling weights and post-stratification to allow for differences in non-response rates based on age group, sex, and census-tract income,” the researchers wrote in the medical journal.

“Using results for both tests, we calculated a seroprevalence range maximizing either specificity (positive for both tests) or sensitivity (positive for either test),” they said.

The researchers noted that seroprevalence was 5 percent by the point-of-care test and 4·6 percent by immunoassay, with no differences by sex and lower seroprevalence in children younger than 10 years.

“There was substantial geographical variability, with higher prevalence around Madrid (over 10 percent) and lower in coastal areas (less than 3 percent),” they said.

The findings suggest that majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas, according to the researchers.

“Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with COVID-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic,” the researchers wrote in the journal.

“These results emphasize the need for maintaining public health measures to avoid a new epidemic wave,” they said.